US6462647B1 - Rechargeable active transponder - Google Patents

Abstract

The present invention concerns an active transponder (30) including an antenna (32) for exchanging a radioelectric signal (34), processing means (36), an accumulator (38) able to supply a first power supply signal (V1), and storage means (40) able to store the power originating from the received radioelectric signal, and to supply a second power supply signal (V2). This transponder further includes: two means (42, 46) for comparing the two power supply signals to a minimum threshold (Vmin) and, in response, providing two control signals (V3, V4); and charging means (50) controlled by the two control signals, so that the accumulator can be charged, via the charging means, from the stored power. One advantage of such a transponder is that the accumulator can be automatically recharged, as soon as the latter is no longer sufficiently charged to assure the functions of the transponder.

Description

The present invention concerns the field of active transponders and, more precisely, active transponders able to be switched into passive transponders.

There conventionally exist a large number.of portable devices for the contactless identification of an unknown object. A portable device of this type contains data able to be transferred, in the form of radioelectric signals, between a fixed base station acting as transceiver and this device. A portable device of this type is usually made using a transponder.

One will recall that there conventionally exist two types of transponders: passive transponders and active transponders.

A passive transponder is arranged for converting a radioelectric signal originating from a base station, into an electric voltage which then supplies the electric power necessary for the transponder's operation.

A passive transponder of this type includes an antenna to be able to receive and transmit radioelectric signals containing data, a storage capacitor connected to the antenna, to be able to store the electric power received by the antenna, and an integrated circuit connected to the antenna and to the storage capacitor, to be able to process the data received by the antenna, and to provide thereto other data to be transmitted in the form of radioelectric signals. It will be noted that, in such a transponder, the storage capacitor supplies electric power to the integrated circuit.

One drawback of such a passive transponder is that contactless interrogation of the transponder requires a small distance between the base station base and the transponder, this distance being of the order of several meters. It is necessary for the passive transponder to be at a small distance from the base station for the transponder to receive sufficient power for its operation. In the event that such a passive transponder is incorporated in a vehicle door key, the user of the key must therefore be a short distance from the vehicle.

An active transponder is arranged for transmitting data over long distances of the order of several hundreds of meters. Indeed, contactless interrogating such an active transponder does not require a short distance between the base station and the transponder, since no transfer of energy occurs from the base station to the transponder.

FIG. 1 shows an active transponder of this type designated by thereference1. This transponder includes an antenna3 arranged so as to be able to receive and transmit radioelectric signals4 containing data, an integratedcircuit5 connected to antenna3, to be able to process data received by antenna3, and to provide the latter with other data to be transmitted in the form of radioelectric signals.Active transponder1 further includes abattery7 able to supply power to the various components oftransponder1.

One drawback ofactive transponder1 is that it operates normally, as long asbattery7 provides sufficient electric voltage to supply the set of components oftransponder1. In the event that the latter is incorporated in a vehicle door key, as soon asbattery7 no longer supplies a sufficient voltage level to assure the proper operation oftransponder1, the latter no longer allows the door opening command to be provided, which is generally inconvenient for the user of the vehicle.

There also exists in the state of the art <<mixed>> or <<dual>> active transponders, i.e. active transponders which can be switched into passive transponders. GB Pat. No. 2,292,866 discloses such a dual transponder.

FIG. 2 of the present description shows adual transponder10 which includes anantenna circuit11, a modulation-demodulation circuit14, arectifier circuit15, a storage capacitor16, abattery18, acentral processing unit20, a read only (ROM)memory21, a random-access (RAM)memory22, aclock system23 and aswitching circuit26.

Antenna circuit11 is arranged for receiving and transmitting data to and from an external device in the form of aradiofrequency wave12. For this purpose,antenna circuit11 includes anantenna11aformed by a coil and acapacitor11b.

Modulation-demodulation circuit14 is arranged for modulating a digital signal received from acontrol circuit25 into an analogue signal, and for demodulating an analogue signal received viaantenna11ainto a digital signal. Thereference13 designates a line via which a received signal is transferred fromantenna11ato demodulation-modulation circuit14, and via which a signal to be transmitted is transferred from modulation-demodulation circuit3 toantenna11a.Rectifier circuit15 is formed by a bridge circuit comprising diodes15ato15dfor converting an alternating voltage fromradiofrequency wave12 received byantenna11ainto a DC voltage V2. Storage capacitor16 is arranged for storing rectified DC voltage V2, this latter being intended to be provided to the different components via aconnection line17.

Central processing unit20 is arranged for processing data in accordance with an execution programme.ROM memory21 is arranged for storing the execution programme.RAM memory22 is arranged for temporarily storing data.Clock system23 is arranged for providing a clock signal in response to whichprocessing unit20 operates. Thereference24 designates a bus via which data and addresses are transmitted.

Battery18 is arranged for providing an electric voltage V1 intended to supply the various components oftransponder10, via aconnection line19. It will be noted that the operating voltage oftransponder10 is of the order of 5 volts.

Switching circuit26 is connected tolines19 and17, and is arranged so that, when voltage V2 across the terminals of storage capacitor16 is greater than voltage V1,transponder10 operates by using the electric power supplied by storage capacitor16, vialine17, and so that, when voltage V2 is less than voltage V1,transponder10 operates by using the electric power supplied bybattery1, vialine19.

One drawback oftransponder10 lies in the fact that, oncebattery18 has run down, the electric power supply of the components oftransponder10 can only be supplied by storage capacitor16. In the event thattransponder10 is incorporated in a vehicle door key, the transponder must be situated at a short distance from the base station so as to receive sufficient power from the base station to operate. In other words, oncebattery18 has run down, the user of the key has to wait the time necessary for capacitor16 to charge, before identification can occur, which can be particularly inconvenient for the user.

An object of the present invention is to provide a dual transponder which overcomes the drawbacks of conventional active transponders, in particular an active transponder which allows a power supply level sufficient to assure its operation to be maintained.

Another object of the present invention is to provide an active transponder which answers the conventional criteria in the semiconductor industry as to complexity and cost.

These objects, in addition to others, are achieved by the active transponder according toclaim 1.

The invention concerns an active transponder including in particular first and second means for comparing respectively the first and second supply signals to a minimum threshold provided by first supply means and, in response, to supply respectively first and second control signals and charging means controlled by the first and second control signals, and arranged so that, when the first and second supply signals are respectively less than and greater than the minimum threshold, the accumulator is charged, via the charging means, from the energy contained in the storage means.

One advantage of such an arrangement of the charging means and the first and second comparison means is that the accumulator charge is controlled, as soon as the supply voltage becomes less than the minimum threshold.

According to another feature of the transponder according to the present invention, the minimum threshold corresponds to the minimum level of supply voltage necessary to assure all the functions of the transponder, which has the advantage of supplying the transponder with sufficient power to assure all the functions of the transponder.

Another advantage of the comparison of the first supply voltage to such a minimum threshold is that it prevents the accumulator being completely run down during the operation thereof, which avoids short-circuiting the processing means.

According to another feature of the transponder according to the present invention, the first comparison means can compare the first supply voltage to a maximum threshold which represents the upper accumulator charge limit, which has the advantage of avoiding overloading the accumulator.

According to another feature of the transponder according to the present invention, the processing means are arranged for receiving the electric power from the storage means when the accumulator is charging, and from the accumulator once it is charged. One advantage of such an arrangement of the processing means is that centralised control of the electric power supply of the transponder components can be achieved.

These objects, features and advantages of the present invention, in addition to others, will appear more clearly upon reading the detailed description of three preferred embodiments of the invention, given solely by way of example, with reference to the annexed drawings, in which:

FIG. 1 which has already been cited, shows a conventional active transponder;

FIG. 2 which has already been cited, shows a conventional dual transponder;

FIG. 3 shows a first preferred embodiment of the active transponder according to the present invention;

FIG. 4 shows in detail a component of the active transponder of FIG. 3;

FIG. 5 shows a second embodiment of the active transponder according to the present invention;

FIG. 6 shows a curve illustrating the relationship between two electric voltages present in the active transponder of FIG. 5;

FIG. 7 shows a third embodiment of the active transponder according to the present invention; and

FIG. 8 shows a timing chart of an electric voltage present in the active transponder of FIG.5.

FIG. 3 shows a first preferred embodiment of the active transponder according to the present invention, designated by thereference30.

Transponder30 includes anantenna32, processing means36, anaccumulator38 and storage means40.

Antenna32 is arranged so as to be able to receive and transmit data to and from an external device (for example a conventional base station), in the form of aradiofrequency signal34.Antenna32 is therefore preferably made by using a coil connected across twoconnection terminals320 and321, in a known manner.

Processing means36 further include twoconnection terminals360 and361 connected respectively toterminals320 and321 ofantenna32. Processing means36 are arranged so as to be able to receive fromantenna32 the received data, process such data, and to supplyantenna32 with identification data to be transmitted in the form of a radiofrequency signal. For this purpose, processing means36 include a logic block which may include a central processing unit (not shown in FIG.3), memory means (not shown in FIG.3), and a clock system (not shown in FIG.3).

Said central processing unit, memory means and clock system are preferably made in the same way as the components described in relation to FIG.2.

Processing means36 are also arranged so as to be able to convertradiofrequency signal34 into a power supply able to provide a DC voltage V2. For this purpose, processing means36 include an interface including a modulation-demodulation circuit (not shown in FIG.3), a rectifier circuit (not shown in FIG. 3) and a terminal364 connected to storage means40.

Said modulation-demodulation circuit and said rectifier circuit are preferably made in the same way as the components described in relation to FIG.2.

Storage means40 are also preferably made by forming a capacitor including a terminal400 connected toterminal364 of processing means36, and anearth terminal401 connected to the earth oftransponder30, so that the voltage present acrossterminals400 and401 is equal to DC voltage V2. By way of illustration only, storage means40 can contain an electric charge equivalent to a voltage of the order of 2 to 6 volts.

Processing means36 include anearth terminal362 connected to the earth oftransponder10, and asupply terminal363 intended to receive a supply voltage V1 supplied byaccumulator38.

Accumulator38 is arranged so as to be able to supply voltage V1, this latter having to be sufficient to supply electrically all the components oftransponder30. For this purpose,accumulator38 includes asupply terminal380 for supply voltage V1, and anearth terminal381 connected to the earth oftransponder30.Accumulator38 is preferably made using a conventional accumulator.

Transponder30 further includes two comparison means42 and46, and charging means50.

Comparison means42 are arranged so as to be able to compare electric voltage V1 supplied by the accumulator to a predetermined minimum threshold voltage Vmin and, in response, to supply an electric voltage V3 so that, when voltage V1 is less than threshold Vmin, voltage V3 is equal to a voltage level <<0>> and so that, when voltage V1 is greater than threshold Vmin, voltage V3 is equal to a voltage level <<1>>.

For this purpose, comparison means42 include afirst input terminal420 connected to supply means44, for receiving threshold Vmin, and asecond input terminal421 connected toterminal380 ofaccumulator38, for receiving voltage V1. Comparison means42 also include anoutput terminal422, for supplying voltage V3 containing the result of the comparison between voltage V1 and threshold Vmin.Output terminal422 of comparison means42 is connected to charging means50, as is described hereinafter.

Comparison means42 are preferably made using an operating amplifier, in a known manner.

Supply means44 include anoutput terminal440 connected toterminal420 of comparison means42, to supply threshold Vmin, as well as a supply terminal and an earth terminal, these latter not being shown in FIG.3. Supply means44 are arranged so that the value of threshold Vmin is sufficient to answer predetermined criteria as to the proper operation oftransponder30, i.e. to determine whether the voltage value supplied by the accumulator is sufficient for the electric supply oftransponder30, i.e. to assure all the functions oftransponder30.

Supply means44 are preferably made using a conventional reference voltage source.

Like comparison means42, comparison means46 are arranged so as to be able to compare electric voltage V2 present acrossterminals400 and401 of storage means40 to threshold Vmin and, in response, to supply an electric voltage V4 so that, when voltage V2 is less than threshold Vmin, voltage V4 is equal to voltage level <<1>> and so that, when voltage V2 is greater than threshold Vmin, voltage V4 is equal to voltage level <<0>>For this purpose, comparison means46 include afirst input terminal460 connected toterminal440 of supply means44, for receiving threshold Vmin, andsecond input terminal461 connected toterminal400 of storage means40, for receiving voltage V2. Comparison means46 also include anoutput terminal462, for supplying electric voltage V4 containing the result of the comparison between voltage V2 and threshold Vmin.Output terminal462 of comparison means46 is connected to charging means50, as is described hereinafter.

Comparison means46 are preferably made using an operational amplifier, in a known manner.

Charging means50 are arranged so as to be able to chargeaccumulator38 from the power stored in storage means40 (i.e. from voltage V2), in response to the comparisons of voltages V1 and V2 to threshold Vmin (i.e. in response to voltages V3 and V4).

For this purpose, charging means50 include afirst input terminal500 connected toterminal400 of storage means40, for receiving voltage V2, and second andthird input terminals501 and502 respectively connected tooutput terminals422 and462, for receiving respective voltages V3 and V4. Charging means50 also include anoutput terminal503 connected toterminal380 ofaccumulator38, via which charging ofaccumulator38 occurs, as is described hereinafter.

FIG. 4 shows charging means50 ofactive transponder30 of FIG. 3 in detail.

As FIG. 4 shows, charging means50 include acurrent mirror52 able to supply, viaterminal503, a charging current I1, acurrent source54 able to supply a first reference current I2, afirst switch56 connected toterminal501, so as to be able to be controlled by voltage V3, and asecond switch58 connected toterminal502, so as to be able to be controlled by voltage V4.

Current mirror52 is connected toterminal500, viaswitch58, so that voltage V2 acts as supply voltage forcurrent mirror52.Current mirror52 is also connected tocurrent source54, viaswitch56, so that current I2 can be recopied bycurrent mirror52 in the form of current I1.

Current mirror52 is preferably made from two field effect transistors T1 and T2, in a known manner. Likewise,current source54 is made from a current mirror formed of two field effect transistors T3 and T4, this mirror receiving a second reference current Iref supplied by a temperature stabilised current source (not shown in FIG.4). Moreover, switches56 and58 are made by forming respectively two field effect transistors T5 and T6 controlled by voltages V3 and V4 respectively.

It will be noted in FIG. 4 that transistors T1, T2, T5 and T6 each have a P type channel, and that transistors T3 and T4 each have an N type channel.

The operation of charging means50, in response to the supply of control voltages V3 and V4 at therespective terminals501 and502 thereof, will now be briefly described.

With reference again to FIG. 4, let us consider a first situation in which voltage V3 is equal to level <<1>>. As a result, transistor T5 is blocked andcurrent source54 is thus not connected tocurrent mirror52. Let us further assume that voltage V4 is equal to level <<1>>. As a result, transistor T6 is not blocked, andcurrent mirror52 is then supplied by voltage V2 present atterminal500.

Let us now consider a second situation in which voltages V3 and V4 are each equal to level <<0>>. As a result, transistors T5 and T6 are conductive, andcurrent source54 is then connected tocurrent mirror52 which is itself supplied by voltage V2. Consequently,current mirror52 supplies charging current I1, viaterminal503.

Those skilled in the art will note that one advantage of the arrangement of charging means50 and comparison means42 and46 intransponder30 is that charging ofaccumulator38 is controlled?, as soon as voltage V1 supplied by the latter becomes less than threshold Vmin, i.e. as soon asaccumulator38 can no longer assure sufficient electric power supply fortransponder30. In other words,active transponder30 has the advantage of being able to rechargeaccumulator38 automatically, as soon as the latter is no longer sufficiently charged to assure all the functions oftransponder30.

The set of components oftransponder30 are preferably made by forming an integrated structure via known CMOS type manufacturing process. Typically, in the aforementioned respective preferred embodiment, processing means36, storage means40, comparison means42 and46 and charging means50 are made in a monolithic manner in a single semiconductor substrate. By way of variant, in addition to these components,antenna42 can also be made in a monolithic manner.

By way of alternative embodiment ofactive transponder30 of FIG. 3, FIGS. 5 and 7 show twoactive transponders60 and70, respectively. It will be noted that the structures oftransponders60 and70 are close to that oftransponder30. Thus, the elements oftransponders60 and70 which are identical to those described in relation to FIG. 3 have been designated by the same references.

As FIG. 5 shows,transponder60 further includes switching means62 and additional supply means63, these components being connected to comparison means42 to form a double threshold comparator, in order to avoid overloadingaccumulator38, as is described hereinafter.

Supply means63 includes anoutput terminal630 connected to switching means62, to supply a maximum voltage threshold Vmax, as well as a supply terminal and an earth terminal, these latter not being shown in FIG.5. Supply means63 are arranged so that the value of threshold Vmax represents the upper charge limit ofaccumulator38. Supply means63 are preferably made using a temperature stable reference voltage source.

Switching means62 are arranged to be able to supply to comparison means42 a threshold Vth which is equal either to threshold Vmin, or to threshold Vmax, as a function of the result of the comparison of voltage V1.

For this purpose, switching means62 include twoinput terminals620 and621 respectively connected toterminal630 of supply means63 and toterminal440 of the supply means, for receiving respectively maximum threshold Vmax and minimum threshold Vmin. Switching means62 further include a control terminal connected toterminal422 of comparison means42, for receiving voltage V3, and anoutput terminal623 connected toterminal420 of comparison means42, for supplying threshold Vth.

Switching means62 are preferably made using two conventional transmission gates.

The operation of the arrangement of comparison means42 and switching means62 will be briefly described.

FIG. 6 shows acurve65 illustrating the relationship between voltage V1 supplied byaccumulator38 and voltage V3 supplied by comparison means42, this curve including six segments <<A>> to <<F>>.

Let us consider a first situation in which voltage V1 increases or decreases so as to be less than threshold Vmin. In this situation, voltage V3 is equal to a first voltage level designated <<0>>, as is shown by segment <<A>> of FIG.6.

Let us now consider a second situation in which voltage V1 increases so as to be greater than threshold Vmin, but less than threshold Vmax. In this situation, voltage V3 is equal to level <<0>>, as is shown by segment <<B>> of FIG.6.

Let us now consider a third situation in which voltage V1 reaches threshold Vmax. In this situation, voltage V3 increases abruptly from level <<0>>, to a second voltage level designated <<1>>, as is shown by segment <<C>> of FIG.6.

Let us now consider a fourth situation in which voltage V1 increases or decreases so as to be greater than threshold Vmax. In this situation, voltage V3 is equal to level <<1>>, as is shown by segment <<D>> of FIG.6.

Let us now consider a fifth situation in which voltage V1 decreases so as to be less than threshold Vmax, but greater than threshold Vmin. In this situation, voltage V3 is equal to level <<1>>, as is shown by segment <<E>> of FIG.6.

Let us now consider a sixth situation in which voltage V1 decreases so as to pass threshold Vmin. In this situation, voltage V3 decreases abruptly from level <<1>> to level <<0>>, as is shown by segment <<F>> of FIG.6.

Those skilled in the art will note that such an arrangement of switching means62 of comparison means42 constitutes a double threshold comparator. Those skilled in the art will also note thattransponder60 forms an improvement totransponder30 of FIG. 3, sincetransponder60 takes account of the upper charge limit ofaccumulator38, which advantageously provides security for the entirety of the accumulator.

It goes without saying that a double threshold accumulator of this type can be made by different components to those described in relation to FIG. 5, while achieving the same function as that described in relation to FIG.6.

With reference now totransponder70 of FIG. 7, processing means36 further includes twoinput terminals365 and366 respectively connected tooutput terminal422 of comparison means42 and tooutput terminal462 of comparison means46, and anoutput terminal367 connected toterminals501 and502 of charging means50.

Moreover, processing means36 of FIG. 7 are programmed so as to be able to supply, viaterminal367, a control voltage V5 to charging means50, in response to electric voltages V3 and V4 received by therespective terminals365 and366. It will be noted that such a control of charging means50 is achieved in an identical manner to the control described in relation to FIGS. 3 and 4. For this purpose, control voltage V5 includes first and second bits containing the respective states of voltages V3 and V4.

One advantage of such an arrangement of processing means36 is that it enables centralised control of the electric power supply of the components oftransponder70 to be achieved. Processing means36 can be arranged so as to be able to be connected to: storage means40 viaterminal364, whenaccumulator38 is being charged (i.e. when voltages V3 and V4 are equal to respective levels <<0>>, so that the electric power necessary for supplyingtransponder70 is supplied by storage means40; andaccumulator38 viaterminal363, onceaccumulator38 is charged (i.e. when voltage V3 is equal to level <<1>>), so that the electric power necessary for supplyingtransponder70 is supplied byaccumulator38.

Those skilled in the art will note that, whenaccumulator38 is being charged,transponder70 operates in the same way as a passive transponder. Indeed, the electric power supply is then provided by storage means40 which contains the power received byantenna32 fromradiofrequency signal34 transmitted by said base station.

The operation oftransponder60 will now be described, with reference again to FIGS. 4 to6 described hereinbefore, and with the aid of FIG. 8 which shows fourtiming charts81 to84 illustrating the temporal evolution of voltages V1 to V4 respectively.

Let us consider the initial situation in which voltage V1 supplied byaccumulator38 is equal to threshold Vmax, as illustrated by timingchart81 between instants t0 and t1. In this case, voltage V3 supplied by comparison means42 is equal to level <<1>>, between instants t0 and t1.

Let us assume now that instant t1 corresponds to an instant from whichtransponder60 is in the magnetic field of a base station which transmits data and energy in the form ofradiofrequency signal34. In other words, from instant t1 onwards,antenna32 oftransponder60 receives data and energy in the form ofradiofrequency signal34.

From instant t1 onwards, storage means40 receive said energy fromantenna32, via processing means36, so that voltage V2 increases until it passes threshold Vmin at an instant t2. As a result, at instant t2, voltage V4 supplied by comparison means46 passes from level <<1>> to level <<1>>. After instant t2, voltage V2 continues to increase to a maximum level corresponding to the maximum charge that storage means40 can store. Consequently, after instant t2, voltage V4 is equal to level <<0>>.

From instant t1 onwards,transponder60 also receives said data fromantenna32, and processes such data, which requires consumption of electric power. This translates into a drop in the level of voltage V1, from instant t1 onwards. As long astransponder60 is within said magnetic field, and is operating, the level of voltage V1 continues to decrease until it reaches threshold Vmin, at an instant t3.

From instant t3 onwards, voltage V1 continues to decrease and becomes less than threshold Vmin, while voltage V2 is greater than level Vmin. As a result, at instant t3, voltage V3 passes from level <<1>>, to level <<0>>. As a result charging current I1 is supplied toaccumulator38, as has already been described hereinbefore with reference to FIG.4. In other words, from instant t3 onwards, charging means50 are controlled to effect charging ofaccumulator38. This results in an increase in voltage V1, and a drop in voltage V2.

Those skilled in the art will note that, on the one hand, storage means40 supply the power to charging means50 to chargeaccumulator38 and that, on the other hand, they receive power from the base station, viaantenna32.

At an instant t4, voltage V1 increases and reaches threshold Vmax, which translates into the passage of voltage V3 from level <<0>> to level <<1>>. In other words, at instant t4, the charge stored inaccumulator38 is maximum, andcurrent source54 of charging means50 is no longer connected to current mirror52: the charging ofaccumulator38 by chargingmeans50 has finished.

Solely by way of illustration, the transponder according to the present invention can advantageously be used in a vehicle door key, this latter being able to identify the key from a distance. With reference again to FIG. 5,transponder60 can operate within the scope of this application in accordance with three different modes: <<rest>>, mode, <<identification>> mode and <<charging>> mode.

In the <<rest>> mode (i.e. in the event that the key is several hundred meters from the vehicle), no data or energy transfer occurs betweenantenna32 and the vehicle. In this case,accumulator38 is only very slightly discharged. This mode is close to the situation described in relation to timingchart81 between instants t0 and t1.

In the <<identification>> mode (i.e. in the event that the key is in proximity to the vehicle), data is exchanged between the key and the vehicle, in the manner of a conventional active transponder. This results in a considerable consumption of electric power, andaccumulator38 is discharged. This mode is close to the situation described in relation to timingchart81 between instants t1 and t3.

In the <<charging>> mode (i.e. in the event that the key is in intimate contact with the vehicle), power is transferred from the vehicle to the key, which allowsaccumulator38 to be charged via chargingmeans50. This mode is close to the situation described in relation to timingchart81 between instants t3 and t4.

It goes without saying for those skilled in the art that the above detailed description can undergo various modifications without departing from the scope of the present invention. By way of variant, the transponder charging means according to the present invention can be made by forming field effect transistors each having a channel whose type of conductivity is opposite to that described in relation to FIG.4.

Claims (12)

What is claimed:

1. An active transponder including:

an antenna arranged so as to be able to receive and transmit a radioelectric signal containing data;

processing means connected to said antenna, and arranged so as to be able to provide said antenna with identification data to be transmitted in the form of radioelectric signals;

an accumulator arranged so as to be able to supply a first electric power supply signal to the components of said transponder; and

storage means arranged so as to be able to store the electric energy originating from said radioelectric signal received by said antenna, and to supply a second electric power supply signal, this transponder further including:

first and second means for comparing respectively said first and second power supply signals to a minimum threshold provided by first supply means and, in response, providing respectively first and second control signals; and

charging means controlled by said first and second control signals, and arranged so that, when said first and second power supply signals are respectively less and greater than said minimum threshold, said accumulator is charged, via said charging means, from the power contained in said storage means.

2. A transponder according toclaim 1, wherein said minimum threshold corresponds to the minimum electric power supply level necessary to assure all the functions of said transponder.

3. A transponder according toclaim 1, wherein said charging means include a first current mirror able to supply a charging current, a current source able to supply a first reference current, first and second switches controlled respectively by said first and second control signals, and wherein said first current mirror is arranged so as to be able to: be supplied by said second power supply signal, as a function of the state of said first switch; and be connected to said current source, as a function of the state of said second switch, so that, when said first and second switches are conductive, said first reference current is recopied by said first current mirror in the form of said charging current.

4. A transponder according toclaim 3, wherein said first current mirror is formed of first and second field effect transistors, wherein said current source is formed of a second current mirror including third and fourth field effect transistors, and receiving a second reference current supplied by a temperature stabilised current source, and in that said first and second switches are formed respectively of fifth and sixth field effect transistors controlled by said first and second control signals respectively.

5. A transponder according toclaim 4, wherein said first, second, fifth and sixth field effect transistors each have a channel of a first type of conductivity, and wherein said third and fourth field effect transistors each have a channel of a second type of conductivity which is different from said first type of conductivity.

6. A transponder according toclaim 1, further including a double threshold comparator including: said first comparison means and switching means able to receive said first control signal and, in response, to supply said first comparison means with a threshold which is equal either to said minimum threshold or to a maximum threshold provided by second supply means.

7. A transponder according toclaim 6, wherein said maximum threshold represents the upper charge limit of said accumulator.

8. A transponder according toclaim 1, wherein said processing means are arranged so as to be able to receive said first and second control signals and, in response, to supply a third control signal able to cause said accumulator to be charged.

9. A transponder according toclaim 8, wherein said processing means are arranged so as to be able to receive the electric power necessary to supply said transponder: from said storage means, when said accumulator is being charged; and from said accumulator once the latter is charged sufficiently.

10. An active transponder according toclaim 1, wherein said processing means, said storage means, said first and second comparison means and said charging means are made in a monolithic manner is a single semiconductor substrate.

11. An active transponder according toclaim 10, wherein said antenna is made in a monolithic manner in said substrate.

12. A portable device including

an active transponder

an antenna arranged so as to be able to receive and transmit a radioelectric signal containing data;

processing means connected to said antenna, and arranged so as to be able to provide said antenna with identification data to be transmitted in the form of radioelectric signals;

an accumulator arranged so as to be able to supply a first electric power supply signal to the components of said transponder; and

storage means arranged so as to be able to store the electric energy originating from said radioelectric signal received by said antenna, and to supply a second electric power supply signal, this transponder further including:

first and second means for comparing respectively said first and second power supply signals to a minimum threshold provided by first supply means and, in response, providing respectively first and second control signals ; and

charging means controlled by said first and second control signals, and arranged so that, when said first and second power supply signals are respectively less and greater than said minimum threshold, said accumulator is charged, via said charging means, from the power contained in said storage means.

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